Schrink film

ABSTRACT

A shrink film comprising a polyethylene film of thickness 5 to 500 μm, wherein the polyethylene comprises an ethylene homopolymer-copolymer mixture having a molecular weight distribution in the range of 5 to 40, and a weight average molecular weight of at least 100 kD.

This invention relates to polyethylene shrink films and to processes fortheir preparation and their uses as packaging materials.

Shrink films are polymer films which on application of heat shrink inone or both directions. They are widely used as packaging and casingmaterials for both large and small products (e.g. industrial pallets,bottles, magazines, etc), generally with thicker films being used forlarger items and thinner films for smaller items.

At present, the most widely used material for shrink film production islow density polyethylene (LDPE), optionally blended with other polymersto achieve a desired balance of properties (e.g. stiffness) and cost.LDPE shrink films may also include a coextruded polypropylene layer toreduce the incidence of fusion of the LDPE layer to shrink film wrappedobjects during the heat treatment to shrink the film. These commonlyused LDPE or LDPE-rich shrink films however suffer from variousproblems. In particular, the shrink film has insufficient mechanicalstrength for many end uses, the occurrence of hole formation during theshrinking process is undesirably high, and the holding force of theshrink film is undesirably low.

Shrink film is produced by extrusion through an annular die with apressure difference applied to blow the extruded cylinder into a filmand achieve the desired orientation within the film, i.e. to build astress into the cooled film. Heat treatment results in stress relaxationand, as a result, shrinkage. Most of the shrinkage occurs while the filmis at its hottest (generally ca. 120-130° C.) during the heat treatment;however the film continues to shrink as it cools. These are referred toas the hot shrink and the cold shrink respectively and for a polymer tofunction adequately as the base material for a shrink film it must meetthe different requirements (in terms of melt strength, cold strength andother mechanical properties) of the hot shrink, cold shrink andpost-shrinkage stages.

We have now found that these requirements are particularly well met,particularly for relatively thin shrink films, by linear low densitypolyethylenes (LLDPE) having a high molecular weight distribution (MWD)and containing an ethylene homopolymer and an ethylene copolymer. (Theterm MWD refers to the ratio (Mw/Mn) between the weight averagemolecular weight (Mw) and the number average molecular weight (Mn) of apolymer).

Thus, viewed from one aspect the invention provides a shrink filmcomprising a low density polyethylene film of thickness 5 to 500 μm,e.g. 20 to 120 μm, preferably 30 to 110 μm, especially 40 to 100 μm,characterized in that said low density polyethylene comprises anethylene homopolymer-copolymer mixture having a molecular weightdistribution in the range 5 to 40, preferably 10 to 35, and a weightaverage molecular weight of greater than 100 kD, e.g. 150 to 300 kD(more preferably 200 to 280 kD, and especially greater than 230, e.g.230 to 270 kD).

Viewed from another aspect the invention provides the use of apolyethylene composition comprising an ethylene homopolymer-copolymermixture having a molecular weight distribution in the range 5 to 40, anda weight average molecular weight of at least 100 kD in the manufactureof a shrink film.

In the shrink film of the invention, the homopolymer componentpreferably has a higher density and lower weight average molecularweight than the copolymer component. Generally, the density of thehomopolymer component should be in the range 960 to 980 kg/m³ while thedensity of the copolymer should be in the range 890 to 920 kg/m³. Theoverall density of the homopolymer-copolymer mixture is preferably inthe range 920 to 945 kg/m³, e.g. 925 to 935 kg/m³, especially 930 kg/m³.

The ratio of homopolymer to copolymer component in the mixture can varywidely. Preferably however, the ratio (by wt) is between 1:10 to 10:1homopolymer to copolymer, especially 1:5 to 5:1, e.g. 1:3 to 3:1. In amost preferred embodiment there should be 40-60% homopolymer to 60-40%copolymer in the mixture. An especially preferred ratio is a slightexcess of homopolymer.

The shrink film of the invention may if desired be a multilayer film,e.g. incorporating a polypropylene protective layer as mentioned above,and a homopolymer-copolymer mixture layer. This layer may also compriseother components besides the homopolymer-copolymer mixture, e.g.coloring agents, other polymers, etc. Generally however such othercomponents will make up no more than 40% wt, more preferably no morethan 25% wt, and especially no more than 10% wt of thehomopolymer-copolymer mixture layer. However, more preferably thehomopolymer-copolymer mixture layer contains no more than 5% wt of anyfurther polymer and ideally this layer consists essentially of thehomopolymer to copolymer mixture.

Viewed from a further aspect therefore the invention provides a shrinkfilm comprising a heat-shrinkable polyethylene layer in which thepolymer content of said layer is at least 95% wt, preferably at least99% wt, and comprising an ethylene homopolymer-copolymer mixture havinga molecular weight distribution in the range 5 to 40 and a weightaverage molecular weight of at least 100 kD (more preferably 150 to 300kD, e.g. 200 to 280 kD, and especially 230 to 270 kD).

In the shrink films of the invention the thickness of thehomopolymer-copolymer mixture layer (i.e the thickness in the non-shrunkfilm) varies depending on whether this forms part of a one layer ormultilayer structure. However, a preferred thickness is 20 to 200 μm,more preferably 40 to 110 μm, especially when the homopolymer/copolymermixture layer forms part of a multilayer film. Accordingly, the films ofthe invention are particularly suitable for wrapping pallets carryingfor example up to 1500 kg loads or wrapping relatively small loads, e.g.having a weight per package of 750 kg or less, more preferably 50 kg orless. Examples of such loads include magazines, books, bottles, sets ofbottles (e.g. with 2 to 12 in a set), etc.

The shrink films of the invention have particularly good properties notjust in terms of their performance during the film shrinking operationbut also in terms of the mechanical properties of the shrunk filmitself. Thus, the shrink film has a particularly beneficial combinationof very low hot shrink force and very high cold shrink force. The lowhot shrink force serves to reduce hole formation during the shrinkingoperation (a major problem with conventional shrink films) while thehigh cold shrink forces provide excellent holding properties, i.e. theyserve to stabilize the shrink-wrapped product. Moreover, the shrunk filmhas mechanical properties (e.g. dart drop and tensile strength) whichare improved relative to conventional shrink films, especially at verylow temperatures. As a result the shrink films of the invention areespecially suited for use in packaging products which will be exposed tolow temperatures during transportation or storage. The excellentmechanical properties of the shrunk film are thought to arise at leastin part from the fact that the polymer is a homopolymer-copolymer ratherthan copolymer-copolymer mixture. This combination of propertiesrelevant to shrink wrapping is both unexpected and highly advantageous.

The MWD of the homopolymer-copolymer mixture is a central parameter inachieving the desired properties of the shrink film. The required MWD of5 to 40, preferably 10 to 35 indicates a broad range of molecularweights for the homopolymer-copolymer mixture. Preferably the MWD is inthe range 15 to 25. Such MWD values can be achieved in a variety of waysall of which are considered to fall within the scope of the invention,e.g. by blending two or more ethylene polymers having differentmolecular weight profiles, by multistage (e.g. two or more stage)ethylene polymerization whereby to generate polyethylenes havingdifferent molecular weight profiles in different polymerization stages,etc. Particularly preferably however the homopolymer-copolymer mixtureis produced by multistage ethylene polymerization using a sequence ofdifferent reactors, e.g. as described in WO 92/12182, preferably atleast one being a slurry loop reactor and another being a gas phasereactor. The two stage sequence loop reactor then gas phase reactor isespecially preferred. The polymerization catalysts used in the ethylenepolymerization may be any catalysts capable of producing a polyethylenewith the desired molecular weight profile, however metallocene and moreespecially Ziegler Natta catalysts are preferred, particularly suchcatalysts in heterogeneous or supported form. Metallocene and ZieglerNatta catalysts capable of appropriate ethylene polymerization are wellknown and are described for example in WO 98/46616 and EP-A-443374.

Where the homopolymer-copolymer mixture is produced as a bimodal polymerin a two-stage polymerization, the initial stage product preferably hasa low molecular weight with a density of at least 960 kg/m³ (e.g. 965 to975 kg/m³) and an MFR_(2.16) (190° C.) of at least 100 g/10 min (e.g.110 to 3000 g/10 min) and the bimodal product preferably has a densityof 920-945 kg/m³ (e.g. 923 to 935 kg/m³), a MFR_(2.16) (190° C.) of 0.05to 1.2 g/10 min (e.g. 0.1-0.8 g/10 min), a weight average molecularweight (Mw) of 150000 to 300000 D (preferably 200000 to 280000D, morepreferably 230000 to 270000D), and a MWD of 10 to 35 (preferably 15 to25).

Such a homopolymer-copolymer mixture may be produced according to thetechnique described in WO 99/41310 using a slurry loop reactor followedby a gas phase reactor, with a density of 931 kg/m³, weight averagemolecular weight (Mw) of 240000D, MFR_(2.16) (190° C.) of 0.2 g/10 min,and an MWD of 22.

In such a two-stage polymerization process, the first stage ispreferably an ethylene homopolymerization with the second stage being acopolymerization.

The comonomer used in the copolymer is preferably a C₃₋₁₂ alpha olefinor a mixture of two or more C₃₋₁₂ alpha olefins, e.g. 1-butene,1-pentene, 1-hexene, 4-methyl-1-pentene, 1-heptene, 1-octene, 1-noneneand 1-decene, with 1-butene and 1-hexene being preferred. Hence, theterm “copolymer” as used herein is intended to cover terpolymers aswell. A preferred terpolymer of use in the invention is anethylene/butene/hexene terpolymer. The comonomer incorporation in acopolymerization reaction is preferably 2 to 10% mole relative toethylene, especially 4 to 8% mole.

In the homopolymerization stage, while no comonomer is added it will beappreciated that the ethylene source may contain trace amounts of C₃₊hydrocarbons copolymerizable with ethylene that may thus becomeincorporated into the ethylene homopolymer. It is well understood in theindustry that the polymer product is nonetheless considered to be anethylene homopolymer.

For film formation using a polymer mixture it is important that thedifferent polymer components be intimately mixed prior to extrusion andblowing of the film as otherwise there is a risk of inhomogeneities,e.g. gels, appearing in the film. Thus, particularly where thehomopolymer-copolymer mixture is produced by blending, it is especiallypreferred to thoroughly blend the components, for example using a twinscrew extruder, preferably a counter-rotating extruder.

Examples of further polymer materials which may be incorporated into thehomopolymer-copolymer mixture layer in the shrink films of the inventioninclude ethylene homo-and co-polymers, and high pressure (HP) copolymers(e.g. ethyl butyl acrylate (EBA), ethyl methacrylate (EMA), and ethylvinyl acetate (EVA) copolymers). Typically these may be included as upto about 40% wt of the homopolymer-copolymer mixture layer, moregenerally up to about 25% wt, e.g. 18 to 22% wt.

The inclusion of polymers such as LDPE, EMA, EVA, and EBA may be used tobalance the shrinkage behaviour of the shrink film in the machinedirection (MD) and the transverse direction (TD). Of these, the HPcopolymers such as EMA, EVA and EBA are preferred as elasticity may beimproved and impact strength may be maintained or improved.

The shrink films of the invention may advantageously be multi-layerfilms, e.g. laminates or coextruded multi-layer films. These multi-layerfilms may be produced by conventional techniques. The layers (other thanthe linear low density ethylene homopolymer-copolymer mixture layer) mayfor example comprise LDPE, Ziegler Natta LLDPE, metallocene LLDPE,ethylene copolymers, polypropylene, and non-woven fabric. Externalpolypropylene and fabric layers may be used to prevent fusion to ordamage to the items being packaged by the shrink film.

The shrink film of the invention will typically be produced by extensionthrough an annular die, blowing into a tubular film by forming a bubblewhich is collapsed between nip rollers after solidification. This filmcan then be slit, cut or converted (e.g. gusseted) as desired.Conventional shrink film production techniques may be used in thisregard. Typically the homopolymer-copolymer mixture layer will beextruded through a die at a temperature in the range 160° C. to 240° C.,and cooled by blowing gas (generally air) at a temperature of 10 to 50°C. to provide a frost line height of 2 to 8 times the diameter of thedie. To obtain balanced shrink properties, the blow up ratio shouldgenerally be relatively high, e.g. in the range 2 to 5.

The shrink films of the invention may of course be used to wrap orencase items, e.g. books, magazines, bottles, etc and this forms afurther aspect of the invention.

Viewed from a further aspect therefore the invention provides a processfor wrapping an object comprising applying a shrink film about saidobject and shrinking said film by the application of heat thereto,characterized in that said film is a shrink film according to theinvention.

Viewed from a further aspect the invention provides an object shrinkwrapped with a shrink film according to the invention.

The shrink films of the invention are distinguished from prior artshrink films in their special combination of improved mechanical andshrink properties. During the film forming process a high degree ofpolymer chain entanglement will occur leading to increased toughness inthe shrink film and to a highly oriented structure which providesimproved shrinkage properties. Thus the films of the invention exhibitthe following advantageous properties: low melt stress (low hot shrinkforce) reducing hole formation during shrinkage; high cold shrink force,leading to better holding by the shrunk film of objects packaged by it;improved mechanical properties for the shrink film, allowing moredemanding (e.g. sharp edged) products to be shrink wrapped and/orallowing thinner films to be used (and hence allowing the amount ofpolymer wrapping to be reduced); and a matt surface. The matt surface ofthe film provides a non-blocking low friction surface without needingthe use of additives to achieve such properties and makes it easier tohandle the film and to open it during the film feeding step of shrinkwrapping. The film is thus easier to use in automated processes.

Thus for a unilamellar shrink film of 150 μm in thickness, the followingproperties are preferred:

Dart drop: at least 650 g/50%

Tear resistance: at least 10N in the machine direction

Cold Shrink Force: at least 350 g, preferably at least 400 in both themachine direction transverse direction Hot shrink force: Less than 8 gin both the machine direction transverse direction.

Moreover for all shrink films of the invention shrinkage in thetransverse direction should preferably be at least 15%, e.g. at least20%.

The dart drop values of the shrink films of the invention areoutstanding and never before have such high Dart Drop values beenobserved in shrink films. Thus, viewed from a still further aspect theinvention provides a polyolefin shrink film, e.g. polyethylene shrinkfilm having a Dart drop value (g)/film thickness (μm) of 5 or more,preferably 5.5 or more, especially 6 or more. Hence, Film 5 in theexamples has a Dart drop value (g)/film thickness (μm) of 700/115=6.09g/μm.

The shrink film in this embodiment should preferably comprise anethylene homopolymer/copolymer mixture, particularly one in which itsmolecular weight distribution is in the range 5 to 40, and its weightaverage molecular weight is at least 100 kD. Such a shrink film ispreferably unilamellar.

We have also found that in unilamellar form (i.e. single layer ratherthan multilayer or laminate form) shrink films formed using thehomopolymer-copolymer mixture may be used for shrink wrapping both smalland large objects and collections of objects. Thus viewed from a furtheraspect the invention provides a unilamellar polyethylene shrink film,characterized in that said polyethylene comprises an ethylenehomopolymer-copolymer mixture having a molecular weight distribution inthe range 5 to 40, and a weight average molecular weight of at least 100kD (more preferably 200 to 280 kD, and especially 230 to 270 kD). Suchfilms may typically have thicknesses of from 5 to 500 μm e.g. 100 to 200μm.

The invention will now be described further with reference to thefollowing non-limiting Examples.

Various terms and properties referred to herein are defined ordetermined as follows:

Molecular Weight Distribution (MWD): This is defined as Mw/Mn where Mwis the weight average molecular weight (in Daltons) and Mn is the numberaverage molecular weight (in Daltons). These are determined by gelpermeation chromatography.

MFR_(2.16) and MFR_(21.6) are melt flow rates determined at 190° C.according to ISO 1133.

Density is determined according to ISO 1183.

The hot and cold shrink forces have been measured in both Machine (MD)and Transverse (TD) directions in the following way. Specimens of 15 mmwidth and 200 mm length are cut out from the film sample in both MD andTD. The samples are tightly mounted into the jaws of the tensile cell insuch a way that the distance between the jaws is 100 mm and the actualforce is zero. The samples are then exposed to hot air at 250° C. in aclosed chamber for 1 minute while the forces are measured. The maximumforce is recorded represents the hot shrink force. The hot air chamberis removed while continuing to record the tensile force. The maximumforce is again recorded and this second maximum represents the coldshrink force.

Shrinkage is measured in both Machine (MD) and Transverse (TD)directions in the following way. Specimens of 10 mm width and 50 mmlength (Li) are cut out from the film sample in both MD and TD. Thesamples are placed on a preheated talcum bed and exposed at 160° C. in aheated oven with circulating air for 2 minutes. After the heat exposure,the residual length (Ls) of the samples is measured.Calculation  of  the  shrinkage  in  MD  (machine  direction/%)$\frac{{L_{i}{MD}} - {L_{s}{MD} \times 100}}{L_{i}{MD}}$Calculation  of  the  shrinkage  in  TD  (transverse  direction/%)$\frac{{L_{i}{TD}} - {L_{s}{TD} \times 100}}{L_{i}{TD}}$Of which:

-   -   L_(i)MD=initial machine direction specimen length.    -   L_(s)MD=machine direction specimen length after shrinkage.    -   L_(i)TD=initial transverse direction specimen length.    -   L_(s)TD=transverse direction specimen length after shrinkage.        Calculation of Hot and Cold Shrink Forces (MD and TD)        S=F/A        Of which:    -   S=Shrink stress (p/mm²)    -   F=Shrink force (p)    -   A=t×b (mm²)    -   b=15 mm    -   t=Average thickness out of three measurements on same sample        (mm)        Impact Resistance (Determined on Dart-Drop (g/50%))        Dart-drop is measured using ISO 7765-1, method “A”. A dart with        a 38 mm diameter hemispherical head is dropped from a height of        0.66 m onto a film clamped over a hole. If the specimen fails,        the weight of the dart is reduced and if it does not fail the        weight is increased. At least 20 specimens are tested. The        weight resulting in failure of 50% of the specimens is        calculated.        Puncture Resistance (Determined in Ball Puncture (Energy/J) at        +23° C., −20° C. and −40° C.)

The method is according to ASTM D 5748. Puncture properties (resistance,energy to break, penetration distance) are determined by the resistanceof film to the penetration of a probe (19 mm diameter) at a given speed(250 mm/min).

Tear Resistance (Determined as Elmendorf Tear (N))

The tear strength is measured using the ISO 6383 method. The forcerequired to propagate tearing across a film specimen is measured using apendulum device. The pendulum swings under gravity through an arc,tearing the specimen from pre-cut slit. The specimen is fixed on oneside by the pendulum and on the other side by a stationary clamp. Thetear strength is the force required to tear the specimen.

Film Thickness Profile (2-Sigma/%)

In the laboratory the film thickness profile is measured by anon-touchable (capacitive) measuring (sensor) system from OctagonProcess Technology. From this measurement one can also get averagethickness, min./max. thickness, standard deviation and calculatedtolerances expressed as 2-Sigma.

Outside and Inside Friction (cof)

Friction is measured according to ISO 8295. By definition it is theresistance to sliding between two surfaces lying in contact with eachother. A distinction is made between Static friction which has to beovercome at the moment sliding motion begins and Dynamic friction whichpersists during a sliding motion at given speed.

EXAMPLE 1

Shrink Films

Three shrink films of 150 μm thickness were prepared by blown filmextrusion as a monolayer film using a conventional film extruder. Theextruder was equipped with a die of diameter 200 mm and die gap 1 mm.Film blowing took place at an extrusion temperature of 200° C., a blowup ratio of 1:3 and a frost line height of 900 mm. Film 1 was producedusing an LDPE (FA3220 available from Borealis A/S) of MFR_(2.16) (190°C.) 0.3 g/10 min and density 923 kg/m³. Film 2 was produced from a blendof 60% wt LDPE (FA 3220) and 40% wt of an LLDPE produced in a solutionprocess having 1-octene as comonomer, MFR_(2.16) (190° C.) 1.0 g/10 min.and density 919 kg/m³. Film 3 was produced using a broad MWDhomopolymer-copolymer mixture according to the invention producedaccording to the technique of WO99/41310 using a slurry loop reactorfollowed by a gas phase reactor, having a density of 931 kg/m³, Mw 240kD, MFR_(2.16) (190° C.) 0.2 g/10 min, and MWD 22. The polymer for Film3 comprised 59% wt of a low molecular weight homopolymer fraction withMFR_(2.16) (190° C.) 300 g/10 min and density 970 kg/m³ and 41% wt of ahigh molecular weight copolymer fraction with MFR_(2.16) (190° C.) <3g/10 min and density <905 kg/m³. The shrinkage and mechanical propertiesof the three films were measured and the results are set out in Table 1below. TABLE 1 Film 1 Film 2 Film 3* Film thickness [μm] 150 150 150Extrusion temperature [° C.] 200 200 200 Blow Up Ratio (BUR) [1:x] 3 3 3Frost Line Height (FLH) [mm] 900 900 900 1% Sec. modulus MD [MPa] 160160 300 1% Sec. modulus TD [MPa] 170 160 330 Free shrinkage at 160° C.MD 67 64 46 [%] Free shrinkage at 160° C. TD 38 30 20 [%] Hot shrinkforce MD [g] 19 11 5 Hot shrink force TD [g] 5 2 1 Cold shrink force MD[g] 340 320 500 Cold shrink force TD [g] 300 330 500 Dart Drop Index[g/50%] 590 640 950 Tear resistance MD [N] 3.4 8.5 20*of the invention

EXAMPLE 2

Shrink Films

Two shrink films of 115 μm thickness were prepared by blown filmextrusion as co-extruded films with a conventional film extruder. Theco-extrusion line was equipped with three extruders and a die having adie diameter of 200 mm and die gap of 1.0 mm. The film blowing tookplace at an extrusion temperature of 200° C. with a blow up ratio of1:3.8 and frost line height of 900 mm.

Film 4 was prepared with all three layers being of the same material asFilm 1. Film 5 was prepared with the outer layers being of the samematerial as Film 1 and the inner layer being of the same material asFilm 3. The layer distribution of Film 5 was 30/40/30% (w/w/w). Theshrinkage and mechanical properties of the two films were measured andthe results are listed in Table 2 below. TABLE 2 Film 4 Film 5* Filmthickness [μm] 115 115 Extrusion temperature [° C.] 200 200 Blow UpRatio (BUR) [1:x] 3.8 3.8 Frost Line Height (FLH) [mm] 900 900 1% Sec.modulus MD [MPa] 150 220 1% Sec. modulus TD [MPa] 160 250 Free shrinkageat 160° C. MD [%] 70 60 Free shrinkage at 160° C. TD [%] 48 36 Hotshrink force MD [g] 18 5 Hot shrink force TD [g] 6 2 Cold shrink forceMD [g] 260 350 Cold shrink force TD [g] 250 340 Dart Drop Index [g/50%]570 700 Tear resistance MD [N] 3.2 11.8*of the invention

EXAMPLE 3

Shrink Film

Three shrink films of 50 μm thickness were prepared by blown filmextrusion as co-extruded films with a conventional film extruder. Theco-extrusion line was equipped with three extruders and a die having adie diameter of 200 mm and a die gap of 1.0 mm. The film blowing tookplace at an extrusion temperature of 200° C. with a blow up ratio of 1:3and a frost line height of 900 mm.

Film 6 was produced with the same material in all layers, namely a LDPEwith MFR_(2.16) (190° C.) 0.7 g/10 min and density 927 kg/³ manufacturedusing a tubular high pressure process.

Film 7 was produced with the surface layers being of the same materialas film 6 while the core layer was produced with the material used forFilm 3. The layer distribution of this film was 25/50/25% (w/w/w).

Film 8 was produced with the same material in all layers, namely thematerial used for Film 3.

The shrinkage and mechanical properties of the three films were measuredand the results are listed in Table 3 below. TABLE 3 Film 6 Film 7* Film8* Film thickness [μm] 50 50 50 Extrusion temperature [° C.] 200 200 200Blow Up Ratio (BUR) [1:x] 3 3 3 Frost Line Height (FLH) [mm] 900 900 900Tensile strength MD [MPa] 32 48 57 Tensile strength TD [MPa] 27 38 44 1%Sec. modulus MD [MPa] 205 250 290 1% Sec. modulus TD [MPa] 215 290 370Free shrinkage at 160° C. MD 78 76 74 [%] Free shrinkage at 160° C. TD26 18 16 [%] Hot shrink force MD [KPa] 145 84 84 Hot shrink force TD[KPa] 11 5 0 Cold shrink force MD [KPa] 1850 2070 2520 Cold shrink forceTD [KPa] 1400 1350 2010 Dart Drop Index [g/50%] 120 340 270 Puncture,Total energy [J] 3.3 6.0 4.6 Tear resistance MD [N] 2.4 1.5 1.5 Haze [%]6.8 9.9 86 Gloss [%] 105 104 6.6*of the invention

1. A shrink film comprising a polyethylene film of thickness 5 to 500μm, wherein said polyethylene comprises an ethylenehomopolymer-copolymer mixture having a molecular weight distribution inthe range 5 to 40, and a weight average molecular weight of at least 100kD.
 2. A shrink film as claimed in claim 1 wherein the homopolymer has adensity of 960 to 980 kg/m³.
 3. A shrink film as claimed in claim 1,wherein the copolymer has a density of 890 to 920 kg m³.
 4. A shrinkfilm as claimed in claim 1, wherein the density of thehomopolymer-copolymer mixture is 920 to 945 kg m³.
 5. A shrink film asclaimed in claim 1, wherein the Mw of the homopolymer-copolymer mixtureis 150 to 300 kD.
 6. A shrink film as claimed in claim 1, wherein the Mwof the homopolymer-copolymer mixture is at least 230 kD.
 7. A shrinkfilm as claimed in claim 1, wherein the MWD of the homopolymer-copolymermixture is in the range 10 to
 35. 8. A shrink film as claimed in claim 7wherein the MWD of the homopolymer-copolymer mixture is in the range 15to
 25. 9. A shrink film as claimed in claim 1, wherein the ratio ofhomopolymer to copolymer in said mixture is in the range 1:5 to 5:1 byweight.
 10. A shrink film as claimed in claim 9 wherein the ratio ofhomopolymer to copolymer in said mixture is in the range 60:40 to 40:60by weight.
 11. A shrink film as claimed in claim 1, wherein thecopolymer comprises ethylene and 1-butene or ethylene and 1-hexene. 12.A shrink film as claimed in claim 1, wherein the copolymer comprises anethylene, 1-butene and 1-hexene terpolymer.
 13. A shrink film as claimedin claim 1, wherein said film has a thickness of 20 to 120 μm.
 14. Ashrink film as claimed in claim 1, wherein said shrink film is amultilayer film.
 15. A shrink film as claimed in claim 1, wherein saidshrink film is unilamellar.
 16. A shrink film as claimed in claim 15having a thickness of 100 to 200 μm.
 17. A shrink film as claimed inclaim 1, wherein said film exhibits at least 15% shrink in thetransverse direction upon application of heat.
 18. A shrink film asclaimed in claim 14 wherein said multilayer film comprises a layer inwhich at least 95% wt is formed from said ethylene homopolymer-copolymermixture.
 19. A process for wrapping an object comprising applying ashrink film about said object and shrinking said film by the applicationof heat thereto, wherein said film is a shrink film comprising apolyethylene film of thickness 5 to 500 μm, wherein said Polyethylenecomprises an ethylene homopolymer-copolymer mixture having a molecularweight distribution in the range 5 to 40, and a weight average molecularweight of at least 100 kD.
 20. An object shrink wrapped with a shrinkfilm comprising a polyethylene film of thickness 5 to 500 μm, whereinsaid polyethylene comprises an ethylene homopolymer-copolymer mixturehaving a molecular weight distribution in the range 5 to 40, and aweight average molecular weight of at least 100 kD.
 21. A method formanufacturing a shrink film which comprises using a polyethylenecomposition comprising an ethylene homopolymer-copolymer mixture havinga molecular weight distribution in the range 10 to 35, and a weightaverage molecular weight of at least 150 kD.
 22. A polyolefin shrinkfilm having a Dart drop value (g)/film thickness (μm) of 5 g/μm or more.23. The shrink film of claim 22 comprising an ethylenehomopolymer/copolymer mixture.
 24. The shrink film of claim 22, whereinthe film is unilamellar.
 25. The shrink film of claim 22, wherein Dartdrop value (g)/film thickness (μm) is 6 g/μm or more.